Can't figure out how to sum only certain dates

Question

Hi all,
Here is my issue: we use numbers to keep a running inventory of pieces in stock, and when a piece sells we mark it with the date sold. I'd love to be able to get a sum of monthly sales, but I can't figure it out. Would someone be kind enough to explain it to me like I was five? Here is a sample of our spreadsheet layout: http://i.imgur.com/sV1RU4s.png
I can easily get a sum for the full column of C, but how do I enter the formula to sum only the sales from April, for instance.
Thanks!!

Answer 1

you can get function descriptions from Apple for free by downloading the function references here:
http://support.apple.com/manuals/#productivitysoftware
you will want to download the formula and function reference.
OK...
sumif() is a function that will sum values in a range based on a condition
sumifs() is a similar function where multiple conditions
both functions look at each row and only sum the values in a particular row when the condition(s) are all met.
=SUMIFS(B,C,">="&E2,C,"<="&EOMONTH(E2, 0))
let's work from the inside out...
EOMONTH() returns the last day of the month you pass if offset by some number of months.  In this case I asked it to give the last day of the same month I passed in so the offset is 0.  The month I passed in is stored in E2.
the basic concept is I only want to sum values in column B when the date in column C is in a paricular month.  said another way... I want to sum values in B where the corresponding date (in the same row) of column C is greater than or equal to the 1st day of the month AND is it also less than or equal to the last to of the month.
SUMIFS() has several arguments a range to sum followed by a pair of arguments (test values range and a condition):
SUMIFS(<SUM_RANGE>, <TEST_RANGE1>, <CONDITION1>, <TEST_RANGE2>, <CONDITION2>, etc )
the sum range is column B
test range 1 is the date column (column C)
the condition is ">="&E1  which concatenates ">=" with the date in cell E1 to make a condition ">=Jan 1, 2013"
test range 2 is the date column (column C)
the condition is "<="&EOMONTH(E1, 0)  which concatenates "<=" with the date corresponding to the last day of the monthe in cell E1 to make a condition "<=Jan 31, 2013"
I hope this helps